1. Field of the Invention
This invention relates to a network system of a time band reservation, a frame transfer method, and a network device.
2. Description of the Related Art
Real-time communication technology such as Institute of Electrical and Electronic Engineers (IEEE) 1394 employs a transfer system (referred to hereinbelow as “cyclic transfer”) using a cycle including real time data and best effort data. FIG. 14 shows a standard cycle pattern. As shown in FIG. 14, cycles are repeated by taking predetermined 125 μs as one cycle. Packet data, that is, a frame, occupying a predetermined band within this one cycle is transferred between network devices. Here, the first half of one cycle is taken as a reserved transfer interval and the second half is taken as a free transfer interval.
The reserved transfer interval is used for real time data communication. In the reserved transfer interval, for example, as shown in FIG. 14, a predetermined time, that is, time bands 1 to 5 are reserved for frame transmission. The reserved time bands 1 to 5 are used only between the respective set devices. Where frames A1 to A5 of real time data are arranged in the reserved time bands 1 to 5, a constant amount of data communication is possible within a constant time. A synchronization frame for synchronizing the network devices is disposed in the header of the reserved transfer interval (not shown in the figure).
By contrast, the free transfer interval is used for best effort data communication that has no real time property. In this interval, no band is reserved. For example, as shown in FIG. 14, where a time band 6 of this interval is free during data transfer, a frame B1 is arranged therein and data communication between the devices is performed. Frames B2 to B5 are similarly arranged in respective bands.
For example, a daisy-chain connection composed of network devices 11 to 14 shown in FIG. 15 and a star connection composed of network devices 11, 12, 13, and 15 can be considered as a network configuration that realizes a cyclic transfer. Each network device has a bridge function, and network devices 12, 13, and 15 can transfer a frame transmitted from a network device on one side of the device to a network device on the other side. As a result, communication can be performed by using a bridge function even between the network devices that are not directly connected to each other.
There is a trend to applying the above-described cyclic transfer to Ethernet (registered trademark), which is a Local Area Network (LAN) standard, and high speed and high reliability of data communication with the cyclic transfer are sought for a LAN using the Ethernet (registered trademark).
Japanese Patent Application Publication No. 2002-185491 (JP-A-2002-185491) discloses a feature of reserving network resources among the devices. More specifically, in response to a reservation request message from a first terminal device, each node device determines whether reservation is possible, performs pre-reservation if reservation is possible, and sends a request message to the next device. The second terminal device determines adaptability for data communication in response to the request message and sends a “Reservation Possible” or “Reservation Impossible” response message to the preceding device. Upon receiving the “Reservation Possible” response message, each node device changes the pre-reservation to main reservation. In this case, whether reservation is possible is determined based on whether or not a reservation resource amount exceeds a resource amount allocated to each node device.
A case in which the method disclosed in JP-A-2002-185491 is applied to the cyclic transfer will be explained below. A case will be considered in which data are transferred from the network device 11 to the network device 14 via the network devices 12 and 13 shown in FIG. 15. In this case, as shown in FIG. 15, a transmission port C1 of the network device 11 is connected to the network device 12 by a cable. Likewise, a transmission port C2 of the network device 12 is connected to the network device 13 and a transmission port C3 of the network device 13 is connected to the network device 14 by a cable.
FIG. 16 shows an example of transfer cycle in the transmission ports of each device shown in FIG. 15. Let us focus attention on a reserved frame (hatched portion) transmission from the transmission port of each device. The reserved time in each device differs from that in another device although one reserved transfer is ensured for one cycle in each device. This is because in the method disclosed in JP-A-2002-185491, reservation is performed by determining whether reservation is possible based on the resource amount when a reservation message is received from a preceding device. Because a delay time required to process the message and a delay time caused by a cable between the devices occur in each device, the transfer is performed with a delay of the reservation request message itself. As a result, as shown in FIG. 16, each device has different reserved time. This delay increases with the increase in the number of relay devices from the device that is a frame transmission source to the device that is a transmission destination, that is, with the increase in the hop number.
In the configuration shown in FIG. 16, the reservation cycle of port C3 differs from those of C1 and C2. In this case, the delay is equal to or greater than one cycle. With certain data to be transmitted, it is desirable that the transfer be performed within one cycle, and this is difficult to do with the transfer method using the reservation described in JP-A-2002-185491.